It's very good, IMO, pivot. The section on noise is very good. Valve specs generally quote Req, which is the noise equivalent resistance and depends just on the spacing and shape of electrodes, mostly on Gm. This relates to the noise which is hiss. This equivalent noise resistance is specced as though the resistor (which doesn't actually physically exist) is at room temperature. All noise of this form is due to random statistical motion of charge carriers in conductors, which has a set form against temperature. In this case, it's not that parts of the valve get hot which makes it any worse, the temperature of a vacuum has no meaning after all !! In this case, the resistance is 'virtual', but behaves as if it is real, including random variation and collisions of charge carriers, and hence noise.
The noise which Hanuman describes is flicker noise, and is a non-ideal, and not part of the equivalent noise resistance. I have seldom encountered it to any extent in valves, it has a physical basis in composition and construction of the cathode, and one might forsee it at very high or low anode currents. It has the same maths as random variations in flames, and friction.
The third type of noise, partition noise, doesn't happen in triodes. So not in this case.